Science Ocean Carbon and Biogeochemistry Studying marine biogeochemical cycles and associated ecosystems in the face of environmental change Volume 3, Number 3 • Fall 2010 News A Preliminary Carbon Budget for the Gulf of Mexico by Paula G. Coble 1, Lisa L Robbins 2, Kendra L. Daly 1, Wei-Jun Cai 3, Katja Fennel 4, Steven E. Lohrenz 5 1. College of Marine Science, University of South Florida, St. Petersburg, FL 2. U.S. Geological Survey, St. Petersburg, FL 3. Department of Marine Science, University of Georgia, Athens, GA 4. Department of Oceanography, Dalhousie University, Halifax, NS, Canada 5. Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS Introduction bon fluxes, and control mechanisms. water management in both countries, The Gulf of Mexico has been a ma- It is a large, semi-enclosed water body as well as changes in temperature jor source of uncertainty in the North geopolitically shared almost equally and rainfall due to climate change, American carbon budget (Chavez by the U.S. and Mexican governments. will profoundly affect Gulf carbon et al., 2007). Until recently, it was The Gulf of Mexico drainage basin ex- sources and sinks. Nutrient discharge sparsely sampled and poorly charac- tends over roughly 40% of each nation from the Mississippi River has been terized in terms of its air-sea exchange and comprises 33 major river systems. implicated in widespread hypoxia on of carbon dioxide, land-to-ocean car- Thus, large-scale changes in land use the shelf. Figure 1. Preliminary shelf-wide carbon budget for the Gulf of Mexico. DIC = dissolved inorganic carbon, TOC= total organic carbon, POC = particulate organic carbon, DOC = dissolved organic carbon. Colored dots represent levels of uncertainty in data. Red dot = High level of uncertainty (i.e. few data available), yellow dot = medium level of uncertainty, blue dot= low level of uncertainty (i.e. many data available), white dot = no data available. Numbersprac - reported in 1012 g C yr-1. tices and OCB NEWS • Fall 2010 1 Science There exists no prior carbon bud- winter estimates were made with and Fluxes were calculated as rates multi- get for the Gulf of Mexico as a whole, without including the Mississippi plied by the area in each region. although Walsh et al. (1989) presented River influence. Fluxes from land many elements of a nitrogen budget. Recent work in the Mississippi- Riverine inputs are only from data We have constructed a carbon budget Atchafalaya River System (MARS) collected in the U.S.. Cai and Lohrenz in part by converting the Walsh et al. has provided estimates of key carbon (2010) estimated inputs from MARS (1989) nitrogen and chlorophyll fluxes fluxes from land to ocean, primary to be 21, 2.5, and 2.5 Tg C yr-1 for into carbon units and in part by addi- productivity, and air-sea flux of CO2 DIC, DOC, and POC, respectively. A tion of new observations collected or (Cai and Lohrenz, 2010) and also re- recent compilation of USGS and EPA compiled more recently. Overall, the sulted in a preliminary carbon budget STORET data for LA rivers (Rob- new data compare well with estimates limited to the area of the plume (Rob- bins et al., in prep.) reported a much derived from Walsh et al. (1989). bins et al., 2009). lower DIC (0.06 Tg C yr-1) but higher This paper represents the next These previous efforts influenced DOC and POC inputs of 8.9 Tg C yr-1 installment in a series of reports on our division of the Gulf into six for each component. Inputs for all coastal carbon budgets currently in regions: 1) Texas coast (TX), 2) Loui- other U.S. Gulf Coast rivers combined preparation as part of the Coastal siana coast (LA), 3) west Florida shelf are 0.86, 2.65, and 1.11 Tg C yr-1 for Carbon Interim Synthesis Activity, (WFS), 4) Mexico (MX), 5) Central DIC, DOC, and POC, respectively. a joint endeavor of the Ocean Carbon Gulf, and 6) MARS Plume. The total The compiled data set suffers from 6 and Biogeochemistry Program and area of the Gulf of Mexico is 1.6 x 10 discontinuous time-series, a range of 2 the North American Carbon Program. km of which the Central Gulf region, methods used in studies, as well as The first installment was “Carbon with depths greater than 200 m, from a dearth of records for which all Budget for the Continental Shelf of composes roughly 63%. The remaining carbon system data were collected. It is the Eastern United States: A Prelimi- area, with depths of 0-200 m, was allo- likely that the DIC estimates compiled nary Synthesis” by Najjar et al. (2010) cated to the coastal regions as follows: from these existing data sets are too published in the Winter 2010 OCB TX - 5%, LA - 11%, WFS - 11%, and low, considering the reports of high Newsletter. This was followed by MX - 11%, all as % total area and thus DIC from agricultural liming prac- “The Marine Carbon Cycle of the adding up to 100%. tices influencing Mississippi River Arctic Ocean: Some Thoughts About Carbon fluxes of interest discharge (Raymond et al., 2008). We the Controls on Air-Sea CO2 Ex- have used values of 22, 11.5, and 10 Tg We considered the following fluxes changes and Responses to Ocean C yr-1, respectively for DIC, DOC, and across three of the four key interfaces: Acidification” by Mathis and Bates POC to obtain a total input from land land-ocean, air-sea, and sediment- published in the Spring/Summer of 43.5 Tg C yr-1. 2010 OCB Newsletter. This current water. No estimates are yet available installment is still preliminary, with for fluxes across the shelf break. Air-sea fluxes major uncertainties and missing Inputs from land are dissolved organic A recent synthesis (Chavez et al., estimates of key fluxes. It is intended carbon (DOC), particulate organic 2007) suggests that the Gulf of Mex- as a status report on the activity with carbon (POC), and dissolved inorgan- ico is a major source for CO2 to the atmosphere (“+” flux) and may domi- the hope that it will stimulate further ic carbon (DIC). Air-sea fluxes are CO2 thought and research. exchange and input of rainwater DOC nate the net flux of the entire North (not included). Sedimentary fluxes American margin because of the The domain are POC deposition, DOC and DIC Gulf’s large size and strong observed The most comprehensive compila- exchange across the sediment-water fluxes, both positive and negative. tion of historical biogeochemical interface (not included), benthic pro- However, these conclusions were based flux data is presented in Walsh et al. ductivity, groundwater inputs of DOC on a very limited data set. Several (1989). They combined a 2-layered and DIC (not included), and hydrocar- projects in the last three years have baroclinic circulation model with a bon seeps. Shelf break fluxes are those greatly increased the number of ob- 21-layered biochemical model to esti- for DIC, DOC, and POC (none are servations and led to the development mate nitrate input, “new” production, included). Internal fluxes are primary of a model incorporating satellite particle export, and remineralization production, respiration (not includ- imagery for sea surface temperature in three coastal areas (west Florida, ed), net community production (not and salinity. However, there are still Louisiana, and Texas) and for the included), and export from the upper not enough data available to reliably Central Gulf. Seasonal, summer and (euphotic) layer. estimate fluxes in the six regions used OCB NEWS • Fall 2010 2 Science for the other processes. Lohrenz et al. tive sign, we have increased the flux 1989; Cai and Lohrenz, 2010; Lohrenz (2010a) measured both positive and for the entire Gulf by 50% to include and Verity, 2006) ranged from 30 to negative fluxes in the northern shelf the influence of the southern Gulf. 3060 g C m-2 yr-1 for the Central Gulf region during three seasons, rang- Carbon fluxes from the upper layer and the Plume regions, respectively. -2 -1 ing from -1.18 to -0.97 mmol m d The model of Walsh et al. (1989) Values from Walsh et al. (1989) were for April 2006, and from 2.71 to 3.22 calculates particle export from the converted from “new” production to -2 -1 mmol m d for October 2005. Values upper layer of the water column net production using an f value (ratio for August 2004 were slightly positive. and from the lower layer (i.e., to the of “new” production to total produc- Robbins et al. (2010a-c) measured over sediment) in units of mg chl m-2 yr-1. tion) of 0.1. These values agreed well 5100 underway data points of pCO2, These were converted to carbon using with those of Lohrenz and Verity TCO2, and pH on the west Florida a C/Chl ratio of 50:1. Values from (2006) for the Central Gulf and TX, shelf (WFS). These data were used Redalje et al. (1994) are available only but were 2 to 3 times higher for the to calculate pCO2 fluxes, showing for the LA and MARS Plume regions, WFS and MX regions. Values for the generally positive fluxes (source) in and are roughly six times higher than plume obtained by Cai and Lohrenz the summer and negative fluxes (sink) the Walsh et al. model results. We (2010) were also lower than those in the winter. Data from 2009 cruises used the average value from Walsh et calculated by Walsh et al. (1989) by a indicate that the WFS was generally a al.